1. Field of the Invention
The present invention relates to a testing apparatus and method for MIMO systems. In particular, the invention relates to a testing apparatus and method for M×N MIMO (Multiple Input Multiple Output) systems that generate signals equivalent to those transmitted through M×N propagation channels and apply the generated signals to a mobile terminal as a test target, such as a mobile phone utilizing the MIMO scheme, or a circuit board contained therein, or an integrated circuit mounted on the board, in order to check them. More specifically, the invention relates to a testing apparatus and method for MIMO systems, in which while the M×N MIMO scheme is maintained theoretically, a technique for reducing the circuit scale of a signal processing circuit indispensable to the modulation technique of the test target is applied.
2. Description of the Related Art
In mobile terminals such as mobile phones and smartphones, there is a demand for high-speed communication of a large amount of information such as image information. To meet this demand, the MIMO scheme is utilized as a scheme of communication between base stations and mobile terminals.
The MIMO scheme is known as a scheme for simultaneously transmitting, from the base station side through M antennas, L sequences of data (L is a number of sequences of data called a layer number) as a transmission signal modulated by a predetermined modulation technique (e.g., OFDM, OFDMA or WCDMA), and receiving the transmission signal at the mobile terminal side through N antennas to thereby separate (demodulate) L sequences of data from the transmission signal.
In the MIMO scheme, transmission data rate of L (L is at most value of min {M, N}) times the transmission data rate of a standard 1×1 communication scheme can be realized, wherein “min {M, N}” indicates one of the M and N, the one of the M and N being lower than the other of the M and N, and L can at most be the lower number of them.
In the MIMO scheme, the M antennas of the base station are each connected to the N antennas of the mobile terminal by the M×N propagation channels. The state of each propagation channel differs depending upon, for example, the locations between the base station and the mobile terminal, and their attitudes. In accordance with the state of each propagation channel, the signals received through the N antennas vary.
In the MIMO scheme, in order to correctly separate L sequences of data from N received signals that vary in level in accordance with the states of the propagation channels, the states of the propagation channels are estimated by transmitting already known signals (pilot signals) from the base station side, and performing a particular calculation using the known signals and the signals actually received at the mobile terminal side. Further, data that enables the transmitter side to perform information transmission of highest quality in the estimated channel states is determined and sent to the base station side.
The data used for signal processing at the transmitter side is called a precoding vector. In the MIMO scheme using codebook based precoding, a set of precoding vectors assumed to be used for the M×N propagation channels is beforehand prepared at the base station side, and transmission information is processed using the precoding vector reported from the mobile terminal side, thereby improving the quality (e.g., signal intensity) of the signal received at the receiver side (mobile terminal).
In view of this, in a test method for a test target, such as a mobile terminal utilizing the M×N MIMO scheme, a circuit board incorporated therein, or an integrated circuit mounted on the board, it is necessary to determine whether the test target selects an appropriate precoding vector corresponding to the characteristics of M×N propagation channels, when reference signals are applied to the M×N propagation channels whose characteristics are already known, and their outputs are applied to the test target.
As an apparatus used for the above-mentioned test, a terminal testing apparatus 10 having the configuration shown in FIG. 9 can be supposed.
The terminal testing apparatus 10 conforms to a MIMO scheme of M=4 and N=2, and a transmission information generating unit 11 incorporated therein generates and outputs four (=M) transmission information signals s0-s3. The transmission information signal is a source signal unique to the modulation technique. For instance, in the case of the OFDMA modulation used in LTE or the OFDM modulation technique used in wireless LAN, the transmission information signal is symbol data for each subcarrier. The transmission information generating unit 11 directly outputs the symbol data, or processes the symbol data using one of the precoding vectors prestored in a codebook (not shown) and outputs the resultant data.
The four transmission information signals s0-s3 are input to signal processing units 121-124, respectively, where they are subjected to signal processing unique to the modulation technique. For instance, if the modulation technique is the aforementioned OFDM, transmission information signals s0-s3 corresponding to subcarriers (in the frequency domain) are transformed into signals in the time domain by an inverse Fourier transformer (IFFT) 12a. The resultant signals are sent to a CP (cyclic prefix) inserting unit 12b, where the copy of a part of its own signal is inserted into the signals. The outputs signals of the inserting unit 12b are sent to a bandpass filter (BPF) 12c, where the signals are transformed into OFDM modulation signals s0′ to s3′ by band limitation processing. Also in other modulation techniques, signal processing corresponding thereto is required to be performed on each signal sequence.
These four modulated signals s0′ to s3′ are sent to a 4×2 channel processing unit 13, in which eight pseudo propagation channels that connect the four input ports to the two output ports are defined, and propagation channel information items h11-h24 indicating the characteristics of the pseudo propagation channels are defined. The propagation channel information items are defined by a parameter setting unit 14.
The 4×2 channel processing unit 13 outputs, through one of the two output ports, a signal r1 obtained by multiplying the input signals s0′ to s3′ by the four propagation channel information items h11-h14 concerning the four propagation channels connected to the one output port. Similarly, the 4×2 channel processing unit 13 outputs, through the other output port, a signal r2 obtained by multiplying the input signals s0′ to s3′ by the four propagation channel information items h21-h24 concerning the four propagation channels connected to the other output port. These two signals r1 and r2 are applied to a test target 1.
Namely, the signals r1 and r2 are expressed as follows:r1=h11·s0′+h12·s1′+h13·s2′+h14·s3′r2=h21·s0′+h22·s1′+h23·s2′+h24·s3′
The test target 1 has a function of measuring propagation channel characteristics based on the input signals r1 and r2, selecting an appropriate precoding vector corresponding to the measured characteristics, and outputting and reporting a codebook index value corresponding to the precoding vector. Accordingly, it is regarded as one of the operation confirmations of the test target 1 in the testing method to check whether the test target 1 selects an appropriate precoding vector for a target propagation channel whose characteristics are already known.
Jpn. Pat. Appin. KOKAI Publication No. 2009-171502, for example, discloses a technique of testing a mobile terminal utilizing the MIMO scheme.
To form propagation channels of the 4×2 MIMO scheme, the testing apparatus disclosed in the above publication needs logical circuit resources corresponding to four transmission antennas, more specifically, needs four signal processing units 121 to 124 that perform signal processing unique to the modulation technique on the four information signals output from the transmission information generating unit 11. In the apparatuses demanded to be reduced in cost, the cost required for the four signal processing units 121 to 124 cannot be ignored.
Further, it is difficult to modify existing testing apparatuses that have a testing function corresponding to the 2×2 MIMO scheme, so as to conform to, for example, the 4×2 MIMO scheme in which a greater number of transmission antennas are used.